VO2（A）納米桿的水熱合成、生長機(jī)理及光學(xué)特性

魏　寧，金　城，熊狂偉，趙　慧，金紹維

（安徽大學(xué)物理與材料科學(xué)學(xué)院，安徽合肥230601）

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VO2（A）納米桿的水熱合成、生長機(jī)理及光學(xué)特性

魏寧，金城，熊狂偉，趙慧，金紹維＊

（安徽大學(xué)物理與材料科學(xué)學(xué)院，安徽合肥230601）

摘 要：用V2O5和草酸作為初始原料，在水熱條件下合成了VO2（A）納米桿.通過改變初始的V2O5與草酸摩爾比以及反應(yīng)時(shí)間制備純相VO2（A）.樣品的元素組成、微結(jié)構(gòu)及光學(xué)性能分別被X射線光電子能譜（XPS）、X射線衍射（XRD）、掃描電鏡（SEM）、差示掃描量熱法（DSC）和傅里葉變換紅外光譜（FT-IR）表征.實(shí)驗(yàn)結(jié)果表明合成VO2（A）的最佳條件為：反應(yīng)溫度230℃、保溫24h以及V2O5對(duì)草酸摩爾比為1∶1.5.結(jié)合XRD數(shù)據(jù)與SEM圖像，提出一個(gè)轉(zhuǎn)變、自組裝和重結(jié)晶過程解釋了VO2（A）納米棒的形成過程.

關(guān)鍵詞：釩氧化物；VO2（A）；水熱合成；相轉(zhuǎn)變；紅外光譜

Received date：2015-07-31

Foundation item：Supported by the National Science Foundation of China（11174001，51402002），the Science Foundation of Anhui Education（KJ2013A030）

Author’s brief：WEI Ning（1987-），male，born in Jinan of Shandong Province，master degree candidate of Anhui University；＊JIN Shaowei（corresponding author），professor of Anhui University，doctoral student supervisor，E-mail：jinsw＠m(xù)ail.ustc.edu.cn.

0　Introduction

Vanadium dioxides，VO2is a typical binary compound with various polymorphs.The polymorphic configurations in this system include rutile-type VO2（R）［1］，monoclinic VO2（M）［2］，tetragonal VO2（A）［3］，monoclinic VO2（B）［4］，tetragonal VO2（C）［5］，etc.Among all of VO2，more effort has been paid to rutile type VO2（R）because it undergoes a fully reversible metal-to-insulator transition（MIT）at 340K［2］，which is closer to the ambient temperature compared to other compounds having thermochromic behavior that had been found［6］.Associated with this transition is a structural distortion from a high temperature，rutile structure VO2（R）（P42／mnm，136）to a low temperature，monoclinic form VO2（M）（P21／c，14）［2］，which has been studied abundantly for applications to smart windows［7-9］.Except for the polymorphic of VO2（R）and VO2（M）structures，the metastable polymorphs of VO2，such as VO2（B），VO2（A），etc.are not frequently reported［10］. In recent years，VO2（B）has gained increasing attention because it is a potential candidate for the cathode in lithium cells for electric vehicles.Studies revealed the layer structure of VO2（B）has high Li＋intercalation performance［11-12］.However，another layer structure of the metastable VO2（A）has limited study until now.One main reason is that the metastable of VO2（A）is usually missed during the preparation of VO2polymorphs.

VO2（A）was firstly reported by Th obaldin the hydrothermal process of V2O4-V2O5-H2O system，the crystal structure of VO2（A）had not been clarified until 1998［13］.The thermal stability of VO2（A）indicates that VO2（A）transforms reversibly from low-temperature tetragonal VO2（A）（P4／ncc，130）to high-temperature body-center tetragonal VO2（AH）（I4／m，87）structure at 162℃. Recently，the synthesis condition of VO2（A）has been studied by some researchers via a hydrothermal route.However，Ji et al.［14］used oxalic to reduce V2O5by a hydrothermal route to synthesis VO2（A）at 270℃.The filling ratio of the experimental procedure in Li’s group［15］was 80%.For ultra-long VO2（A）nanorods，Liu et al.［16］reported the hydrothermal treatment time was 48h.Thus a simple and effective method for preparing metastable of VO2（A）nanostructures is very essential for theoretical research and practical applications.

In this paper，VO2（A）nanorods with about 10μm in length are prepared by one pot hydrothermal process in V2O5-H2C2O4-H2O system.The phases of VO2（B）and VO2（A），as well as their transformation can be controlled by varying both the molar ratio of V2O5to oxalic acid and reaction time at a lower temperature of 230℃for 24hwith a filling ratio of 40%.The microstructure，phase transition and optical properties of the VO2（A）samples were carefully studied. A possible growth mechanism was proposed to explain the formation process of the VO2（A）nanorods.

1　Experimental section

1.1 Synthesis

Vanadium pentoxide and oxalic acid（H2C2O4·2H2O）are of analytical grade and used without any further purification.In a typical synthesis，0.3g of V2O5（orange yellow）powder and 0.21-0.42 g of H2C2O4·2H2O were dispersed into 20mL of distilled water with vigorous stirring for about 30 min at room temperature.After，the mixed solution was transferred into a Teflon-lined autoclave（50 mL）with stainless steel shell（the filling ratio designated as fin the captions of the following figures is 0.4），which was sealed and sustained at 230℃for 2-60h，and then cooled to room temperaturenaturally.The precipitates were collected by filtering，washed with distilled water and ethanol alternately，and dried in an oven at 70℃for 10hfor further characterization and measurement.

1.2 Characterization

X-ray photoelectron spectroscopy（XPS，ESCALAB-250）was used to identifiy the composition of the sample and the valence of the vanadium.The phase structure was determined by X-ray diffraction （XRD）with Philips X’Pert diffractometer（Cu Kαradiation，λ＝1.540 6）at 40kV and 30mA. The morpholgys of the as-obtained samples were examined using scanning electron microscopy（SEM，S-4800）.Differential scanning calorimetry（DSC）analysis were performed using a Q2000under a nitrogen gas flow with a scan rate of 10℃·min-1.Optical properties of VO2（A）nanorods were measured by Fourier transform infrared spectroscopy（FT-IR，Nicolet 8700）with an adapted heating controlled cell.

2　Results and discussion

Fig.1shows the typical XPS spectrum of the sample prepared at 230℃for 24hwith a molar ratio of 1∶1.5（V2O5／oxalic acid）.No other element peaks except for those of V，O and C are observed on the survey spectrum（Fig.1a）.The peak of C1sis ascribed to some carbon dioxides absorbed on the surfaces of the sample and can be disregarded.The spectrum of Fig.1bshows the peaks at 516.6，524.3and 530.4eV are attributed to the V2p2／3，V2p1／2and O1slevel，they are characteristics of vanadium in＋4oxidation state and accord with the values of bulk VO2reported in literatures［17-18］.For the obtained sample，theΔ（O1s-V2p2／3）value is 13.8eV，which is true of the reported value of V4＋in literature［18］，this confirms the vanadium valence to be＋4oxidation state.

Fig.1　XPS spectra of as-obtained VO2powders sample：survey spectrum（a），core-level spectrum of V2pand O1s（b）

Generally，the reaction temperature，reaction time，and the initial molar ratio of vanadium source to reducing agent under the hydrothermal conditions are greatly important for the VO2synthesis.Fig.2 shows the XRD patterns of the samples synthesized at temperature of 230℃after 24hwith different molar ratios of V2O5／oxalic acid（from 1∶1.2to 1∶1.8）.The samples obtained at a molar ratio of 1∶1.2can be indexed to the metastable VO2（B）phase（JCPDS：31-1438）［19］，it has gained increasing interest as a cathode material for lithium cells.Upon increasing the molar ratio to 1∶1.5，the isolated sample can be indexed to pure phase VO2（A）（JCPDS：42-0876）［3］.As depicted in Fig.2c，the most remarkable peaks belong to the（hk0）family，which strongly suggests a preferential growth along a specific orientation.When the molar ratios are raised to 1∶1.6and／or 1∶1.8，the obtained samples appear to be a mix of the original VO2（A）along with a prominent proportion of VO2（B）.Such results reveal the initial molar ratio of V2O5／oxalic acid plays an important role in synthesizing high purity ofVO2（A）.As shown in Fig.2，the preparation of phase-pure VO2（A）is only achieved in hydrothermal treatment at 230℃for 24hwith a molar ratio of 1∶1.5（V2O5／oxalic acid），where as the mix of VO2（A）and VO2（B）is observed below and above this optimum molar ratio.

Fig.2　XRD patterns of the samples obtained at 230℃after 24h with varying molar ratios of V2O5to oxalic acid（f=0.4）

The SEM images of the samples corresponding to Fig.2（XRD）are depicted in the Fig.3. Analyzing the SEM images（Fig.3）and comparing the results of XRD（Fig.2），one can draw the conclusion that the VO2（B）appeared as the short belt-like structures（Fig.3a），the VO2（A）appeared as the long rod-like structures（Fig.3c）.

Fig.3　SEM images of the series samples obtained at 230℃after 24hwith varying molar ratios of V2O5／oxalic acid（f=0.4）：1∶1.2（a）；1∶1.4（b）；1∶1.5（c）；1∶1.6（d）

Fig.4shows the XRD patterns of the samples obtained at 230℃for different reaction times under a molar ratio of 1∶1.5（V2O5／oxalic acid），which reveal the formation and evolution of the VO2（A）phase.From the diffraction pattern of the Fig.3a，it was observed that all peaks of the isolated sample after the reaction of 2hbelong to the VO2（B）phase.Upon increasing reaction time to 12h，the samples appear to be mix of the primary VO2（B）along with a noticeable proportion of the VO2（A）.Further increasing the time to 36hor longer（such as，60h），the samples appear to be exclusively phase-pure of VO2（A）.This suggests that the VO2（B）formed at 230℃after 2h，and then the VO2（A）phase was transformed from VO2（B）with the increase of the time.Eventually，the pure VO2（A）formed after the reaction of 24h（Fig.3c）.Galy［20］proposed that the phase transition VO2（B）→VO2（A）is just a crystallographic slip Cs＝1／3［-100］（001），occurring in the median plane of the double layers assembled by VO6octahedra of the VO2（B）［4］.

Fig.4　XRD patterns of the samples prepared after different amounts of time at 230℃with a 1∶1.5molar ratio of V2O5to oxalic acid（f=0.4）

The morphological evolution of the samples prepared at 230℃for different reaction times under a molar ratio of 1∶1.5was depicted in Fig.5.The morphology of VO2（B）（Fig.5a）is short belt after the reaction of 2h.With futher reaction，some short VO2（B）belts combine together，and assemble to the VO2（A）nanorods（Fig.5b）.This combination occurs continuously，and the VO2（A）nanorods progressively elongate with increasing time to 36h（Fig.5c）.Finally，the long VO2（A）nanorods with lengths up to tens of micrometers are formed after the reaction of 60h（Fig.5d）.Several benched surfaces and short rods attached to long rods can be clearly observed（Figs.3cand 5d）.These are evidence of the self-assembled nanorods combining or attaching.

Fig.5　SEM images of the samples prepared at 230℃at a molar ratio of 1∶1.5（V2O5／oxalic acid）with various reaction time：2h（a）；12h（b）；36h（c）；60h（d）

Through the XRD patterns（Fig.4）and SEM images（Fig.5），it is found that the belt-like VO2（B）is the intermediate product to synthesize VO2（A）nanorods.A possible process for the formation of VO2（A）nanorods was schematically charted in Fig.6.（1）VO2（B）belts are fast formed by the hydrothermal reaction between V2O5and H2C2O4，as described in Figs.4aand 5a.（2）VO2（B）short belts transformed and assembled to VO2（A）nanorods，as depicted in Figs.4b-c and Fig.5b. （3）VO2（A）short rods are attached and recrystallized to long rods.（4）VO2（A）nanorods continue to grow with further prolonging the time（Figs.5c-d）.Briefly，the formation of the VO2（A）nanorods can be described as the transformation，assembly，attaching and recrystallization process.

Fig.6　Schematic of the formation mechanisms for VO2（A）nanorods

Fig.7shows the DSC curves of the obtained samples with different molar ratios.In the DSC scan，the thermal behavior of the samples was performed when it was heated（from 20to 200℃）and then cooled（from 200to 20℃）at 10K·min-1.During the heating process，a single endothermic peak was observed at 168.3℃，it can be assigned to conversion of the primitive tetragonal VO2（A）into a body-centered tetragonal VO2（AH）structure.A wide exothermic peak is detected at 122.8℃on cooling，it is ascribed to the transformation of the VO2（AH）into the VO2（A）form.In previous studies，Oka et al.［21］reported a phase transition of the VO2（A）sample at 162℃only upon heating not on cooling from the DTA and magnetic susceptibility measurements.In our work，the transition temperature of 168.3℃for the VO2（A）nanorods raised about 6℃.The higher transition temperature and wide hysteresis loop can be considered to be due to the nonstoichiometry of VO2（A）nanorods and／or the scaling to nanoscale dimensions，as described in the VO2（M）nanostructures［22］. There are no peaks detected in the heating and cooling process of VO2（B）because no structure transition occurs.The endothermic peak at 169.9℃for sample synthesized with a molar ratio of 1∶1.8may caused by the crystal boundary between VO2（B）and VO2（A）.

Fig.7　DSC curves of the samples on the heating and cooling process

Contrast with the rutile VO2（R），the optical properties of VO2（A）in the infrared region（IR）are limited studied.The infrared spectra of Fig.8shows a clear process of the phase transition in the VO2（A）sample before and after the Tc（Transition temperature）.It is clearly seen that as-prepared VO2（A）nanorods has optical switching property at absorption bands from 680to 660cm-1where its missing can be ascribed to the delocalisation of the electrons involving in the V4＋-V4＋bonds between VO6octahedra［16］.It suggests the VO2（A）has potential application in optical switching devices.Two IR curves below Tc（one from the heating，the other from cooling）are basically coincided，revealing the phase transition of the VO2（A）nanorods has good reversibility.These optical properties indicate that the VO2（A）is a good candidate for the application of an infrared light switching material.

Fig.8　Infrared spectra for the VO2（A）samples under different temperatures

3　Conclusion

VO2（A）nanorods were facilely prepared by hydrothermal approach using V2O5，oxalic acid and H2O as starting materials.The results reveal that the optimal synthesis condition of the VO2（A）nanorods is hydrothermal process at 230℃with a 1∶1.5molar ratio of V2O5to oxalic acid after 24h. An assembly，attaching and recrystallization mechanism is considered to be responsible for the formation of VO2（A）nanorods.An endothermic peak at 168.3℃on heating and an exothermal peak at 122.8℃on cooling were detected in DSC curve for pure VO2（A）product.The infrared spectra indicate that the VO2（A）nanostructures can be used as the optical switching materials at bands from 680to 660cm-1.

References：

［2］ MORIN F J.Oxides which show a metal-to-insulator transition at the neel temperature［J］.Phys Rev Lett，1959，3（1）：34-36.

［3］ OKA Y，YAO T，YAMAMOTO N.Powder X-ray crystal structure of VO2（A）［J］.J Solid State Chem，1990，86（2）：116-124.

［5］ HAGRMAN D，ZUBIETA J，CHRISTOPHER J，et al.A new polymorph of VO2prepared by soft chemical methods［J］.J Solid State Chem，1998，138（1）：178-182.

［6］ MAENG J S，KIM T W，JO G H，et al.Fabrication，structural and electrical characterization of VO2nanowires［J］.Mater Res Bull，2008，43（7）：1649-1656.

［7］ KANG L T，GAO Y F，LUO H J.A novel solution process for the synthesis of VO2thin films with excellentthermochromic properties［J］.ACS Appl Mater Interface，2009，1（10）：2211-2218.

［8］ ZHANG Z T，GAO Y F，CHEN Z，et al.Thermochromic VO2thin film：solution-based processing，improved optical properties，and lowered phase transformation temperature［J］.Langmuir，2010，26（13）：10738-10741.

［9］ DAI L，CAO C X，GAO Y F，et al.Synthesis and phase transition behavior of undoped VO2with a strong nano-sizeeffect［J］.Sol Energy Mater and Sol Cells，2011，95（2）：712-715.

［10］ ZHANG S D，SHANG B，YANG J L，et al.From VO2（B）to VO2（A）nanobelts：first hydrothermal transformation，spectroscopic study and first principles calculation［J］.Phys Chem Chem Phys，2011，13：15873-15881.

［11］ LIU H M，WANG Y G，WANG K X，et al.Design and synthesis of nanothorn VO2（B）hollow microsphere and their application in lithium-ion batteries［J］.J Mater Chem，2009，19：2835-2840.

［12］ MILOˇSEVIC′S，STOJKOVC′I，KURKO S，et al.The simple one-step solvothermal synthesis of nanostructurated VO2（B）［J］.Ceramics International，2012，38（3）：2313-2317.

［13］ OKA Y，SATO S，YAO T，et al.Crystal Structures and transition mechanism of VO2（A）［J］.J Solid State Chem，1998，141（2）：594-598.

［14］ JI S D，ZHANG F，JIN F P.Selective formation of VO2（A）or VO2（R）polymorph by controlling the hydrothermal pressure［J］.J Solid State Chem，2011，184（8）：2285-2292.

［15］ LI M，KONG F Y，LI L，et al.Synthesis，field-emission and electric properties of metastable phase VO2（A）ultra-long nanobelts［J］.Dalton Trans，2011，40：10961-10965.

［16］ LIU P C，ZHU K J，GAO Y F，et al.Ultra-long VO2（A）nanorods using the high-temperature mixing method under hydrothermal conditions：synthesis，evolution and thermochromic properties［J］. CrystEngComm，2013（15）：2753-2760.

［17］ CHEN Y S，XIE K，LIU Z X.Determination of the position of V4＋as minor component in XPS spectra by difference spectra［J］.Appl Surf Sci，1998，133（4）：221-224.

［18］ MENDIALDUA J，CASANOVA R，BARBAUX Y.XPS studies of V2O5，V6O13，VO2and V2O3［J］.J Electron Spectrosc Relat Phenom，1995，71（3）：249-261.

［19］ OKA Y，YAO T，YAMAMOTO N，et al.Phase transition and V4＋-V4＋paring in VO2（B）［J］.J Solid State Chem，1993，105（1）：271-278.

［20］ GALY J.A proposal for（B）VO2（A）VO2Phase transition：a simple crystallographic slip［J］.J Solid State Chem，1999，148（2）：224-228.

［21］ OKA Y，OHTANI T，YAMAMOTO N，et al.Phase transition and electrical properties of VO2（A）［J］.J Ceram Soc Jpn，1989，97（10）：1134-1137.

［22］ WHITT AKER L，JAYE C，F(xiàn)U Z G，et al.Depressed phase transition in solution-grown VO2nanostructures［J］.J Am Chem Soc，2009，131（25）：8884-8894.

（責(zé)任編輯 鄭小虎）

CLC number：O613.51 Document code：A Article ID：1000-2162（2016）01-0042-08

Hydrothermal synthesis，growth mechanism and optical property of VO2（A）nanorods

WEI Ning，JIN Cheng，XIONG Kuangwei，ZHAO Hui，JIN Shaowei＊
（School of Physics and Materials Science，Anhui University，Hefei 230601，China）

Abstract：VO2（A）nanorods were facilely synthesized by using V2O5and oxalic acid as the starting materials via a hydrothermal method.The initial molar ratio of V2O5to oxalic acid and reaction time were changed to obtain the pure VO2（A）phase.The elemental composition，microstructures and optical properties of as-obtained VO2samples were characterized using X-ray photoelectron spectroscopy（XPS），X-ray diffraction（XRD），scanning electron microscope（SEM），differential scanning calorimetry（DSC）and Fourier transform infrared spectrometer（FT-IR）.It was found that the optimal synthesis condition of VO2（A）is hydrothermal process at 230℃for 24hwith a 1∶1.5molar ratio of V2O5／oxalic acid.Combining the XRD patterns and SEM images，a transformation，assembly，recrystallization process was proposed to explain the formation process of VO2（A）nanorods.

Key words：vanadium oxides；VO2（A）；hydrothermal synthesis；phase transition；infrared spectroscopy

doi：10.3969／j.issn.1000-2162.2016.01.008